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使用 Paho C 连接到部署

本文主要介绍如何在 C 项目中使用 Eclipse Paho C ,实现客户端与 MQTT 服务器的连接、订阅、收发消息等功能。

Eclipse Paho C 与 Eclipse Paho Embedded C 均为 Eclipse Paho 项目下的 C 语言客户端库(MQTT C Client),均为使用 ANSI C 编写的功能齐全的 MQTT 客户端。

前置准备

MQTT 服务器部署

  • 使用 EMQX 提供的 免费公共 MQTT 服务器,该服务基于 EMQX 的 全托管 MQTT 云服务 创建。服务器接入信息如下:
    • 连接地址: broker.emqx.io
    • TCP Port: 1883
    • SSL/TLS Port: 8883
  • 您也可以自己创建部署,在部署概览下可以查看到连接相关的信息,请确保部署状态为运行中。使用 TCP 端口或 SSL/TLS 端口 测试连接到 MQTT 服务器。如果您是自己创建部署,请设置认证,在部署控制台认证鉴权 > 认证 中设置用户名和密码,用于连接验证。

安装依赖

bash
sudo apt-get update
sudo apt-get -y install build-essential git cmake

编译 Paho MQTT 库

Linux/Mac

bash
git clone https://github.com/eclipse/paho.mqtt.c.git
cd org.eclipse.paho.mqtt.c.git
make
sudo make install

Windows

mkdir build.paho

cd build.paho

call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x64

cmake -G "NMake Makefiles" -DPAHO_WITH_SSL=TRUE -DPAHO_BUILD_DOCUMENTATION=FALSE -DPAHO_BUILD_SAMPLES=TRUE -DCMAKE_BUILD_TYPE=Release -DCMAKE_VERBOSE_MAKEFILE=TRUE ..

nmake

通过 TCP 端口连接

本节介绍了如何通过 TCP 端口连接到部署。

  1. 导入所需库函数

    c
    #include "stdlib.h"
    #include "string.h"
    #include "unistd.h"
    #include "MQTTClient.h"
  2. 定义连接地址、认证信息以及消息发布和接收主题

    c
    // Enable or disable SSL/TLS connection (1 for SSL/TLS, 0 for TCP)
    #define USE_SSL 0
    
    #if USE_SSL
    #define ADDRESS "ssl://broker.emqx.io:8883"
    #else
    #define ADDRESS "tcp://broker.emqx.io:1883"
    #endif
    
    #define USERNAME    "emqx"
    #define PASSWORD    "public"
    #define CLIENTID    "c-client"
    #define QOS         0
    #define TOPIC       "emqx/c-test"
    #define TIMEOUT     10000L
  3. 开始连接 / 编写 MQTT 代码

    c
    int rc;
    MQTTClient client;
    
    MQTTClient_create(&client, ADDRESS, CLIENTID, 0, NULL);
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    conn_opts.username = USERNAME;
    conn_opts.password = PASSWORD;
    
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        exit(-1);
    } else {
        printf("Connected to MQTT Broker!\n");
    }

通过 SSL/TLS 协议连接

本节介绍了如何通过 SSL/TLS 端口连接到部署。

  1. 调整连接地址,通过将 USE_SSL 设置为1实现。

    c
    // Enable or disable SSL/TLS connection (1 for SSL/TLS, 0 for TCP)
    #define USE_SSL 1
  2. 开始连接 / 编写 MQTT 代码,

    通过函数 configureSSLOptions() 来设置SSL选项。如果需要配置服务器证书,可以通过 ssl_opts.trustStore 加载证书。如果不需要,将 ssl_opts.enableServerCertAuth 设置为0。

    c
    MQTTClient_SSLOptions configureSSLOptions() {
        MQTTClient_SSLOptions ssl_opts = MQTTClient_SSLOptions_initializer;
        ssl_opts.enableServerCertAuth = 1;
        // CA_CERTIFICATE_FILE_Path specify server CA
        // ssl_opts.trustStore = CA_CERTIFICATE_FILE_Path;
        return ssl_opts;
    }
    
    ···
    
    #if USE_SSL
        MQTTClient_SSLOptions ssl_opts = configureSSLOptions();
        conn_opts.ssl = &ssl_opts;
    #endif

订阅和发布

本节主要介绍了如何在已连接到部署的情况下订阅主题并发布消息。

订阅主题

设置将要订阅的主题及对应 QoS 等级

c
// subscribe topic
MQTTClient_subscribe(client, TOPIC, QOS);

取消订阅

通过以下代码取消订阅,此时应指定取消订阅的主题。

c
MQTTClient_unsubscribe(client, TOPIC);

发布消息

发布消息时需要告知 MQTT Broker 目标主题及消息内容。

c
void publish(MQTTClient client, char *topic, char *payload) {
    MQTTClient_message message = MQTTClient_message_initializer;
    message.payload = payload;
    message.payloadlen = strlen(payload);
    message.qos = QOS;
    message.retained = 0;
    MQTTClient_deliveryToken token;
    MQTTClient_publishMessage(client, topic, &message, &token);
    MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Send `%s` to topic `%s` \n", payload, TOPIC);
}

// Call the `publish` function in a loop to finish publishing the message
char payload[16];
for (int i = 0; i < 100; i += 1) {
    // publish message to broker
    snprintf(payload, 16, "message-%d", i);
    publish(client, TOPIC, payload);
    sleep(1);
}

接收消息

通过以下代码指定客户端对消息事件进行监听,并在收到消息后执行回调函数,将接收到的消息及其主题打印到控制台。

c
MQTTClient_setCallbacks(client, NULL, NULL, on_message, NULL);
···
// Define the on_message callback function to print the content of the messages received by the subscribed topic
int on_message(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    char *payload = message->payload;
    printf("Received `%s` from `%s` topic \n", payload, topicName);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}

断开连接

如客户端希望主动断开连接,可以通过如下代码实现:

c
MQTTClient_disconnect(client, TIMEOUT);
MQTTClient_destroy(&client);

以上部分仅列出了一些关键代码,项目完整代码请见:[mqtt-client-C-paho]https://github.com/emqx/MQTT-Client-Examples/tree/master/mqtt-client-C-paho),您可以进行下载并体验。

完整代码

c
#include "stdlib.h"
#include "string.h"
#include "unistd.h"
#include "MQTTClient.h"

// Enable or disable SSL/TLS connection (1 for SSL/TLS, 0 for TCP)
#define USE_SSL 1

#if USE_SSL
#define ADDRESS "ssl://broker.emqx.io:8883"
#else
#define ADDRESS "tcp://broker.emqx.io:1883"
#endif

#define USERNAME    "emqx"
#define PASSWORD    "public"
#define CLIENTID    "c-client"
#define QOS         0
#define TOPIC       "emqx/c-test"
#define TIMEOUT     10000L

MQTTClient_SSLOptions configureSSLOptions() {
    MQTTClient_SSLOptions ssl_opts = MQTTClient_SSLOptions_initializer;
    ssl_opts.enableServerCertAuth = 1;
    ssl_opts.trustStore = CA_CERTIFICATE_FILE;
    return ssl_opts;
}


void publish(MQTTClient client, char *topic, char *payload) {
    MQTTClient_message message = MQTTClient_message_initializer;
    message.payload = payload;
    message.payloadlen = strlen(payload);
    message.qos = QOS;
    message.retained = 0;
    MQTTClient_deliveryToken token;
    MQTTClient_publishMessage(client, topic, &message, &token);
    MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Send `%s` to topic `%s` \n", payload, TOPIC);
}

int on_message(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    char *payload = message->payload;
    printf("Received `%s` from `%s` topic \n", payload, topicName);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}

int main(int argc, char *argv[]) {
    int rc;
    MQTTClient client;

    MQTTClient_create(&client, ADDRESS, CLIENTID, 0, NULL);
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    conn_opts.username = USERNAME;
    conn_opts.password = PASSWORD;

#if USE_SSL
    MQTTClient_SSLOptions ssl_opts = configureSSLOptions();
    conn_opts.ssl = &ssl_opts;
#endif

    MQTTClient_setCallbacks(client, NULL, NULL, on_message, NULL);
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        exit(-1);
    } else {
        printf("Connected to MQTT Broker!\n");
    }
    // subscribe topic
    MQTTClient_subscribe(client, TOPIC, QOS);
    
    char payload[16];
    for (int i = 0; i < 10; i += 1) {
        // publish message to broker
        snprintf(payload, 16, "message-%d", i);
        publish(client, TOPIC, payload);
        sleep(1);
    }
    MQTTClient_unsubscribe(client, TOPIC);
    MQTTClient_disconnect(client, TIMEOUT);
    MQTTClient_destroy(&client);
    return rc;
}

测试验证

  1. 编写 CMakeLists.txt
    cmake_minimum_required(VERSION 3.17)
    find_package(eclipse-paho-mqtt-c 1.3.9 REQUIRED)
    project(mqtt_c C)
    include_directories(/usr/local/include)
    link_directories(/usr/local/lib)
    set(CMAKE_C_STANDARD 99)
    add_executable(mqtt_c main.c)
    target_link_libraries(mqtt_c paho-mqtt3c)
  2. 编译和运行代码 c_mqtt

更多内容

综上所述,我们实现了在 C 项目中创建 MQTT 连接,使用客户端与 MQTT 服务器进行订阅、收发消息、取消订阅以及断开连接的场景。可以在 这里 下载到示例的源码,同时也可以在 GitHub 上找到更多其他语言的 Demo 示例。